In most locomotive traction motor drive systems, the AC output of an engine driven alternator is converted to DC by a full wave power semiconductor bridge assembly to provide a DC supply voltage for energizing the windings of one or more AC or DC traction motors. The rectifier assembly typically includes a relatively large number of power semiconductor devices, and may be conveniently housed within a chamber integral with the housing of the alternator. In this way, the ventilating air provided to cool the alternator may be used to cool the power semiconductor devices as well. Such an arrangement is set forth in principle in U.S. Pat. No. 3,340,413 to Drabik, issued Sep. 5, 1967, and assigned to the assignee of the present invention.
Whether housed at the alternator or at a remote location, the conventional practice in constructing power semiconductor bridge assemblies is to individually mount a number of stud-mount power semiconductor devices in parallel on two or more electrical bus bars, and to electrically interconnect the free current carrying terminals of the devices with the phase windings of the alternator to form a full wave bridge; see the above referenced patent to Drabik. A drawback of this type of arrangement is that removal and replacement of a failed power semiconductor device may be relatively difficult and time consuming. Another drawback is that paralleling hardware and voltage drop matched devices are required.
More recently, there have been efforts to utilize higher current capacity power semiconductors in a disk-type package to minimize the number of devices, essentially to that required by the circuit topology. This also simplifies assembly and maintenance procedures, while improving reliability and controllability.